Pressure vessels are known in which the lid is pressed onto the vessel with the aid of hinged bolts or plug-in bolts. The seal is, in these cases, provided by a vertical compression gasket. This type of fixing of the lid to the vessel is generally used only for vessels with relatively large openings. It also has the disadvantage that its operation requires a considerable amount of time and the tightening of the bolts must be carried out as evenly as possible to provide a good seal.
In order to facilitate the time-consuming operation entailed in opening and closing pressure vessels, central closures are also known. However, such closures are expensive and are usually used only for vessels with large openings.
Bayonet closures are frequently used for closing pressure vessels of small capacity, for example pressure cookers. In these designs, the seal is in most cases provided by lip gaskets and, in some cases, a self-adapting gasket is also used. On closing and opening of these locking devices, however, the resilient gasket rubs against the edge of the vessel, leading to premature wear of the gasket and hence to leakages. The abrasion of both the gasket and the metal parts also causes contamination of the contents of the vessel.
This phenomenon can have dangerous consequences in sterilization vessels for medical instruments. If such small particles, which adhere to the instruments to be sterilized, such as injection needles or injection syringes, enter the human or animal organism when these instruments are used, health may be detrimentally affected.
It is not only when using such pressure vessels for medical or pharmaceutical purposes that such contamination can have an adverse effect; such is also the case when pressure vessels are used in laboratories for research purposes and the like. Furthermore, such lid closures with lip gaskets are only suitable for super-atmosphere pressure vessels.
Usually, vessels designed for super-atmosphere pressures of up to 3 atmospheres gauge must be used to kill germs reliably. This excess pressure is required both for steam sterilizers and gas sterilizers. In addition it is necessary in many cases to extract the air from the pressure vessel prior to the steam sterilization. Gas sterilization makes it absolutely essential to extract air. For such applications, the lid closure must also be pressure-tight when a vacuum is applied.